1. Field of the Invention
The present invention relates to a chain tensioner including a tensioner body having a cylindrical plunger bore with an open end, a cylindrical plunger slidably inserted in the plunger bore, and biasing means accommodated inside an oil pressure chamber formed between the plunger bore and the rear end of the plunger such as to be able to expand and contract and to bias the plunger outward, the chain tensioner being capable of maintaining appropriate tension of a chain.
2. Description of the Related Art
It has been common practice to use a chain tensioner for maintaining appropriate tension in the chain. For example, in a chain guide mechanism that slidably guides a transmission chain such as an endless roller chain passing over respective sprockets of a crankshaft and a cam shaft inside an engine room, by means of a guide shoe, it is known to press a pivoting chain guide having the guide shoe by a chain tensioner to maintain appropriate tension.
For example, as shown in FIG. 13, an endless timing chain CH passes over a drive sprocket S1 attached to a crankshaft and a pair of driven sprockets S2 and S3 attached to cam shafts in an engine room. A chain guide mechanism has a pivoting chain guide G1 and a stationary chain guide G2 for guiding this timing chain CH.
The stationary chain guide G2 is fixed in position in the engine room with two mounting shafts B1 and B2, while the pivoting chain guide G1 is attached such as to be pivotable around the mounting shaft B0 in the plane in which the timing chain CH runs in the engine room.
The chain tensioner 500 maintains the tension of the timing chain CH at an appropriate level as well as reduces its vibration by pressing the pivoting chain guide G1 against the chain.
The known chain tensioner 500 used in such a chain guide mechanism includes, for example, as shown schematically in FIG. 14, a tensioner body 510 having a cylindrical plunger bore 511 with an open end, a cylindrical plunger 520 in the plunger bore 511 to slide against the cylindrical surface 513 of the plunger bore 511, and biasing means that biases the plunger 520 outward from the plunger bore 511.
The biasing means is a coil spring 540 accommodated inside a cylindrical recess 521 in the cylindrical plunger 520 and compressed between the plunger and the bottom 512 of the plunger bore 511.
Oil is supplied from an oil supply hole 514 formed in the plunger bore 511, so that an oil pressure chamber 501 formed between the plunger bore 511 and the plunger 520 is filled with oil, which oil biases the plunger 520 outward. A check valve 550 (schematically shown as a check ball) stops the oil from flowing out from the oil supply hole 514.
As the plunger 520 reciprocates, oil flows through the small gap between the plunger 520 and the plunger bore 511, and the flow resistance provides the damping effect of slowing down the reciprocal movement of the plunger 520.
In such a known chain tensioner 500, when let stand for a long time after the oil supply is stopped (in the case with an engine, after the engine is stopped), there is a time lag before oil is supplied immediately after the next start-up. As no oil is replenished but the oil in the oil pressure chamber 501 leaks out when the plunger 520 moves back and forth, there occurs a shortage of oil in the oil pressure chamber 501. Because of this shortage of oil, the damping force of the oil does not act on the plunger 520, which sometimes lead to vibration and abnormal noise of the timing chain CH, or may damage the timing chain CH.
Therefore, it is also known to design the tensioner to have an oil reservoir chamber inside the plunger to supply oil into the oil pressure chamber. A constant amount of oil is thereby retained in the oil reservoir chamber, so that, even immediately after the start-up after a long stop, oil remaining in the oil reservoir chamber is supplied to the oil pressure chamber, to maintain the damping force of oil on the plunger and to prevent vibration or damage of the chain (see Japanese Patent Application Laid-open No. H9-177907 and Japanese Patent No. 4376278).